Abstract
This work exploits the potential of a novel simultaneous transmission and reflection reconfigurable intelligent surface (STAR-RIS) in improving the security in a multiple-input single-output (MISO) network, where three transmission protocols, namely, energy splitting (ES), mode selection (MS), and time splitting (TS), are studied. The objective is to maximize the weighted sum secrecy rate (WSSR) by jointly designing the beamforming (BF) and the transmitting and reflecting coefficients (TARCs). A path-following based technique is first developed to covert the non-convex problem and design the BF and the TARCs in an alternating manner. Then, for the ES scheme, the TARCs are solved by a penalty concave convex procedure (PCCP) method. Besides, the proposed method is extended to the MS by solving a mixed-integer problem. While for the TS scheme, a two-layer optimization method is proposed. Finally, the simulation results validate the superiority of STAR-RIS.